The present invention relates to a wireless device which automatically selects a better channel when radio interference, or the like, is detected in a network which uses a wireless communication function.
In recent years, the IEEE802.11 wireless LAN system has been a wireless packet communication system standardized as a system which realizes high speed wireless data transfer. In this wireless LAN system, one base station (host device) and at least one terminal (slave device) constitute one wireless link. The wireless devices that constitute this wireless link share one wireless channel based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance).
When the frequency band of 5 GHz is used, 4 channels are available. When the frequency band of 2.4 GHz is used, 3 channels (4 channels in some cases) are available.
In the conventional IEEE802.11 wireless LAN system, a channel designated by a user at the start up of the wireless LAN system is shared, and this channel is used to establish communications unless the user switches the channel. Thus, when the radio wave condition deteriorates in this channel, a data communication fails in some cases.
According to IEEE802.11h standard with which the above problem is solved, the channel may be automatically switched in the case of a deteriorated radio wave condition. Further, there is an obligation to immediately switch the channel when receiving a specific frequency packet (weather radar, aeronautical radio navigation, or the like).
FIG. 2 shows a communication sequence between a base station (host device) and a terminal (slave device) 2. In the example of FIG. 2, the terminal 2 is compliant with IEEE802.11h standard and, when the base station switches the channel, the terminal 2 accordingly performs a channel switch operation. In FIG. 2, the time flows vertically from top to bottom. Firstly, after the start up, both the base station and the terminal 2 operate with Channel_0. In this start-up period, the terminal 2 sends a communication of association request 301 to the base station. In response to the association request 301, the base station returns a signal of association response 302 to the terminal 2. Through the association sequence consisting of the association request 301 issued by the terminal 2 and the association response 302 issued by the base station, communication between these devices is established. Both these associated devices are provided with data services through the same channel. However, when the association condition is deteriorated, the base station searches for a channel of a better condition. For example, the base station finds a channel of a better association condition, Channel_2 (2ch), and switches the channel from Channel_0 to Channel_2. In this case, the base station sends to the terminal 2 a channel switch announcement signal 310 which notifies the terminal 2 about the switch of the channel to a new channel, Channel_2. Thereafter, the base station switches the channel to Channel_2 by itself.
FIG. 3A is a flowchart of the channel switch operation carried out by the base station of FIG. 2. FIG. 3B is a flowchart of a channel switch operation which is carried out by the terminal 2 of FIG. 2 according to the switch of channel in the base station. In the operation of the base station shown in FIG. 3A, after the start up, the base station enters the normal operation state (step S1a). Then, at step S2a, the condition of a new channel candidate is measured in order to switch the currently-selected channel to a new one in case of deterioration of the current channel. Then, at step S3a, it is determined from a result of the channel measurement at step S2a whether or not the condition of the measured new channel candidate is better than that of the current channel. If the condition of the new channel candidate is not better than that of the current channel (“No” at step S3a), the process returns to the channel measurement of step S2a, and the condition of another candidate channel is measured. This series of steps, i.e., measurement and determination, is performed on every one of all the channels till a channel whose condition is better than that of the current channel is found. If it is determined that the condition of the new channel candidate is better than that of the current channel (“No” at step S3a), the base station sends a channel switch announcement signal to the terminal 2 (step S4a) and switches the current channel to a new channel having a better condition (step S5a).
In the operation of the terminal 2 shown in FIG. 3B, after the start up, the association processing of associating with the base station is performed (step S1b). Then, it is determined whether or not the association processing has succeeded (step S2b). If the association processing has failed (“No” at step S2b), the association processing is performed again at step S1b. The series of steps S1b and S2b is repeated till the association processing succeeds. If the association processing has succeeded (“Yes” at step S2b), the terminal 2 enters the normal operation state (step S3b). After entering the normal operation state, the terminal 2 checks whether or not the channel switch announcement signal has been received in order to check whether or not the base station has switched the channel due to deterioration of the radio wave condition, i.e., whether or not the base station has sent the channel switch announcement signal to the terminal 2 at step S4a of FIG. 3A. If the switch announcement is not received (“No” at step S4b), it is determined that a communication can be established with the current channel, and the terminal 2 returns to the normal operation mode of step S3b. This check is performed at every predetermined cycle. When reception of a channel switch announcement is confirmed, the process proceeds to step S5b where the channel is switched to a new channel designated by the channel switch announcement.
In the case where both the base station and the terminal are compliant with IEEE802.11h as illustrated in FIGS. 2, 3A and 3B, the base station and the terminal are associated through Channel_0 in a start-up period, and thereafter, the base station periodically performs the channel measurement. When the radio wave condition is deteriorated, the channel switch announcement signal 310 is sent to the terminal 2, and both the base station and the terminal switch the channel used such that normal data communication is secured.
For example, PCT publication WO 00/16532 discloses a conventional technique for dynamically switching the channel (frequency) in a wireless network (hereinafter, referred to as “dynamic frequency switching technique”). According to this technique, each of the base station and the terminal performs channel measurement. The base station selects a channel having a preferable condition based on results of the measurement and notifies the terminal about the number of the selected channel, whereby dynamic frequency switching is realized.
However, in the case where a base station determines a new channel and notifies terminals about the number of the new channel to switch the current channel as in the above-described conventional dynamic frequency switching technique, a terminal which is included in the wireless network but is not compliant with IEEE802.11h standard cannot identify the channel switch announcement received from the base station. Thus, there is a possibility that this terminal cannot follow the dynamic frequency switching of the base station. This problem is described below with reference to the drawings.
FIG. 1 shows a wireless network formed by a base station (host device) and two terminals (slave devices) 1 and 2, i.e., three devices in total. In FIG. 1A, the terminal 1 and the terminal 2 are associated with the base station through Channel_0. Herein, it is assumed that the base station and the terminal 2 are compliant with IEEE802.11h standard, but the terminal 1 is not. Referring to FIG. 1B, when the base station switches the channel to Channel_2, the terminal 2 which is compliant with IEEE802.11h standard accordingly switches its channel to Channel_2 to be kept associated with the base station. However, the terminal 1, which is not compliant with IEEE802.11h standard, cannot follow the switch of the channel of the base station and remains with the former Channel_0 to be incapable of establishing communication with the base station.
FIG. 4 shows a communication sequence between the base station which is compliant with IEEE802.11h standard and the terminal 1 which is not compliant with IEEE802.11h standard.
In the example of FIG. 4, the base station and the terminal 1 operate with Channel_0 in the start-up period. The terminal 1 issues an association request 401 to the base station, and the base station sends an association response 402 in return, whereby an association is established between these devices, and the normal operation (data services) is started.
In the start-up period, the base station also establishes an association with the terminal 2 (not shown in FIG. 4) which is compliant with IEEE802.11h standard to start data services. In the case where the base station dynamically switches currently-selected Channel_0 to Channel_2 due to a change of the radio wave condition, reception of a specific frequency packet, or the like, the terminal 2 receives a channel switch announcement based on IEEE802.11h from the base station to switch its channel to be the same as that of the base station as illustrated in FIG. 1B. Specifically, as illustrated in FIG. 2, the terminal 2 receives the channel switch announcement 310 based on IEEE802.11h from the base station and switches its channel to the same one according to the announcement 310, whereby data services can be continued.
However, the terminal 1 is not compliant with IEEE802.11h and therefore cannot identify the channel switch announcement 310 based on IEEE802.11h issued from the base station. Thus, as shown in FIG. 1B, the terminal 1 remains with the former Channel_0 to be incapable of establishing communication with the base station.